Many hyperfrequency generators are known using a semi-conductor component as source element.
These are diodes known, for example, under the denominations IMPATT, LSA, tunnel diodes, GUNN effect diodes, and other components of negative resistance.
The emitting diode is installed inside a closed volume connected to the adjoining resonant cavity through a specific adjustable coupler.
According to American patent No. 3,534,293, HARKLESS inventor, the hyperfrequency source element is mounted in its feeding circuit in series with a rectilineal radiating rod coaxially disposed in a cylindrical volume. This radiating rod constitutes the coupling antenna present in a chamber adjacent to a resonant cylindrical cavity.
Because of its role and position, this chamber constitutes a preresonator.
This preresonator includes a transversal output leading to an intermediate adjustable conduit and is connected transversely to a cylindrical resonant cavity of variable volume whose output is connected to the load.
This preresonator presents the characteristics adapted for the envisioned range of frequencies. It assures filtering out undesirable modes.
The coupler is tuned by a screw to the desired frequency to select the modes that will be excited in the resonant cavity.
This type of generator with preresonator has the drawback of a high selectivity as to frequency.
So, these generators cannot furnish, above a certain level, a constant level of power over a large range of frequencies.
However, some applications demand the maintenance of a sufficient level of power over a large range of frequencies while at the same time, presenting a very low degree of phase noise.